1. Field of the Invention
The present invention relates to an antenna having low backside gain and multipath-resistant performance and a receiver using the antenna.
2. Description of the Related Art
For conventional receiving and broadcasting systems using satellites, microwave band radio waves passing through the ionosphere are used. One of the largest problems of the antenna when receiving such radio waves is enumerated as elimination of an influence of multipath.
When the antenna is disposed with its front panel faced in the direction toward the zenith, direct waves from the satellites (right-handed circularly polarized waves) fall from above a horizontal plane and do not enter from below the horizontal plane. Accordingly, it is one of the solutions for eliminating an influence of multipath that all radio waves entered from below the horizontal plane should be taken as reflected waves and such radio waves should not be received. However, in the case of the antenna using a patch antenna and so forth, since the antenna itself also has the sensitivity to the radio waves entered from the backside of the antenna, the antenna is under the great influence of multipath as it is. Therefore, in order to minimize the influence of multipath, reduction of backside gain of the antenna has been demanded so far.
Accordingly, in order to prevent from receiving the radio waves entered at low elevation angles or from below the horizontal plane, a patent document 1 discloses an antenna 100 as shown in FIG. 12. FIG. 12(A) is a perspective view of the antenna 100, and FIG. 12(B) is a sectional drawing of a cross section taken on line A-A in FIG. 12(A).
The antenna 100 comprises a dielectric 101, a radiation conductor 102, a ground conductor plate 103a, a ground conductor plate 103b, and a conductor wall 104. The radiation conductor is provided on the upper surface of the dielectric 101, and the ground conductor plate 103a with a bottom area larger than that of the dielectric 101 is provided on its underside. Furthermore, the ground conductor plate 103b is provided on the underside of the ground conductor plate 103a via the conductor wall 104. Thus, the antenna 100 has a choke structure. The conductor wall 104 is disposed, for example, at a distance of ¼ wavelength of a radio wave received, from edges of the ground conductor plate 103b. Since a radio wave entered from behind can be trapped by this choke structure, it is possible to reduce the influence of multipath on the antenna 100.
Besides, an antenna 200 as shown in FIG. 13 is disclosed in patent documents 2 and 3. FIG. 13(B) is a bottom plan view, and FIG. 13(A) is an end elevation of a cross section taken on line A-A in FIG. 13(B).
The antenna 200 comprises a ground conductor plate 204 having a dielectric 201, a radiation conductor 202, and notches 203. The radiation conductor 202 is provided over almost all area on the upper surface of the dielectric 201, and the ground conductor 204 is provided over almost all area on the underside except for the portions corresponding to the notches 203. Furthermore, the notches 203 are formed nearly in the center of four sides of the ground conductor plate 204.
According to such a configuration, the notches 203 allow a pathway length of the induction current flowing across the outer circumferential edge to be elongated, out of the induction current induced in the ground conductor plate 204, without preventing a main flow of induction current crossing over the center portion. Therefore, it is possible to raise an F-to-B ratio (the ratio of the front side gain to the backside gain) by weakening downward radiation attributable to induction current induced in the ground conductor plate 204.
[Patent document 1] JP-A-2000-77930
[Patent document 2] JP-A-2005-203873
[Patent document 3] Japanese Patent No. 3764289
However, the antenna 200 described in the patent documents 2 and 3 not only can raise the F-to-B ratio, but also brings about a problem of reducing the sensitivity of overall antenna 200. That is to say, since electric current flows much into the ground conductor plate 204 immediately below the radiation conductor 202, the notches 203 provided on the ground conductor plate 204 may serve to reduce the sensitivity of overall antenna. Besides, in the case of a circularly polarized wave antenna, not only the sensitivity of a left-handed circularly polarized wave is reduced, but the sensitivity of a right-handed circularly polarized wave is also reduced more than necessary.
Furthermore, since the notches 203 provided on the ground conductor plate 204 immediately below the radiation conductor 202 cause the antenna 200 to change the frequency characteristics or directional pattern, modification of basic design itself of the antenna 200 may be required to compensate for the above-mentioned change. This problem leads to a fundamental problem that radio waves sent from satellites and so forth cannot be received properly.
The invention is devised to solve these problems, and it is an object of the invention to provide an antenna capable of reducing the backside gain while keeping the sensitivity of overall antenna to a certain level. Particularly, in the case of an antenna subject to a circularly polarized wave, it is an object of the invention to provide an antenna capable of reducing the sensitivity of a left-handed circularly polarized wave more than ever before as well as keeping the sensitivity of a right-handed circularly polarized wave to a certain level. This allows an influence of multipath, which is harmful when receiving signals from satellites and so forth, to be reduced more than ever before.
It is a further object of the invention to provide an antenna, which causes little variation of the frequency characteristics or directional pattern of the antenna as well as requiring no modification of basic design for their compensation, when reducing the sensitivity of the left-handed circularly polarized wave.